Spinal fusion involves joining two or more adjacent vertebrae with a bone fixation device to restrict movement of the vertebrae with respect to one another. For a number of known reasons, spinal fixation devices are used in spine surgery to align and/or fix a desired relationship between adjacent vertebral bodies. Such devices typically include a spinal fixation element, such as a relatively stiff fixation rod that is coupled to adjacent vertebrae by attaching the fixation element to various bone fixation elements, such as hooks, bolts, wires, screws, and the like. The fixation elements can have a predetermined contour and, once installed, the fixation elements hold the vertebrae in a desired spatial relationship, either until desired healing or spinal fusion has taken place, or for some longer period of time.
Recently, dynamic or flexible fixation elements have come into use. Dynamic fixation elements are desirable to permit some movement and shock absorption upon implantation on a patient's spine. In addition, the removal of bone structure, such as facet joints or laminae, result in instabilities of the motion segments of the spine. Consequently, a fixation system should stabilize the motion segment in anteroposterior translation as well as in axial rotation. Both motion patterns result in shear stress within the rods of fixation systems. This is especially important in elderly patients, where the bone quality is sometimes compromised, becoming sclerotic or osteoporotic.
Excessive stiffness of a rod element may cause abnormalities and diseases of the spine, as well as significant discomfort to the patient. Although some existing spinal fixation devices do provide some level of flexibility, what is needed is a dynamic stabilization system that enables a range of flexibilities and stiffnesses to be applied to a patient's spine.